The efficacy of an ophthalmic laser scanning imaging system is very much dependent on the ability of a laser system to establish an effective spot size for its laser beam. To do this, it is very desirable that the system's laser beam converge to a diffraction limited focal spot (i.e. a Diffraction Limited Point Spread Function: DL-PSF). An important aspect for achieving this desired result is that the laser beam will have a substantially flat wavefront. Aberrations that distort the beam's wavefront, however, are frequently (i.e. almost always) introduced into the laser beam. The consequence here is that a distorted wavefront is introduced into the laser beam that, if not corrected, is detrimental to an optimization of the DL-PSF.
For ophthalmic imaging procedures, it happens that the eye itself frequently introduces significant optical and phase aberrations. Specifically, the cornea, the lens and the retina of an eye can each contribute to the introduced aberrations. As indicated above, for an optimal DL-PSF, these aberrations need to either be removed or, at least, minimized.
Using a wavefront analyzer (e.g. a Hartmann-Shack sensor) the total effect of aberrations that may be introduced into a light beam can be measured. The resultant measurements can then be used to create, or to program, devices that will compensate for the introduced aberrations. In each case, the objective is to reconstitute a light beam (e.g. diagnostic light beam of a laser scanning ophthalmoscope) with a substantially flat wavefront. Stated differently, distortions of a wavefront that result from external influences (i.e. the eye) need to be removed by counter-distortions. As mentioned above, this objective (i.e. a resultant flat wavefront) is important for establishing a DL-PSF that will improve the overall efficacy of a resolution and contrast of a laser scanning ophthalmoscope.
It is a well known physical phenomenon that light will be refracted when a wave crosses a boundary between two media in which its phase velocity differs. A consequence of this is an Optical Path Difference (OPD) that corresponds to a phase change in the light beam. Further, the extent of this phase change depends on the distance light travels through a medium. For instance, in a material (e.g. plastic) having a change in refractive index equal to 0.01 (i.e. Δn=0.01), the OPD for light traveling through five microns of the material will be approximately one-tenth of the light's wavelength (OPD=λ/10). The total effect, however, is distance dependent. For example, light traveling through twenty microns of material will have an OPD of 0.4λ. Accordingly, there will be a complete 360° phase shift in λ every fifty microns.
It is also well known that a plastic material can be optically altered by radiation from a femto-second laser beam, with consequent results in OPD as described above. Specifically, it can be shown that a single burst of a femto-second laser beam (e.g. pulses generated at 50 MHz for 100 femto-seconds) focused to a spot in the plastic material, will alter the material through a depth of about five microns. And, this alteration will result in an OPD of around λ/10. Furthermore, for a “k” number of aligned bursts, the OPD will equal approximately kλ/10. Depending on how the spots are arranged, various OPDs can be combined to affect the wavefront of a light beam in a predetermined manner. Thus, as implied above, when selected refraction changes are introduced into a light beam having a distorted wavefront, the result can be a return of the wavefront to a “flat” or “plane” configuration.
In light of the above, it is an object of the present invention to provide a system and method for removing aberrations from a light beam to improve convergence for an improved Diffraction Limited Point Spread Function. Another object of the present invention is to provide a phase plate for use in a retinal imaging system that is customized for the particular patient. Still another object of the present invention is to provide a customized phase plate that effectively compensates for static aberrations that are introduced into a light beam by the patient's eye. Yet another object of the present invention is to provide a customized phase plate for use with an ophthalmic laser system that is easy to use, is simple to manufacture and is relatively cost effective.